In the field of post-trauma and post-operative physical therapy for the rehabilitation of joints, it is generally known that the occurrences of capsular, ligamentous and articular adhesions, thromboembolisms, venous status, post-traumatic osteopenia, peripheral edema, muscle atrophy and the like can be reduced by an early mobilization of the injured or surgically treated joint with a continuous passive motion device (CPM).
Various continuous passive motion devices have been proposed for effecting post-trauma or post-operative movements of an injured or surgically treated limb or joint. These devices are often driven by an electric motor and operate to continuously and repeatedly flex the affected or associated joint at a predetermined speed and through a predetermined range of motion. Moreover, these devices may be specially adapted for use in conjunction with a particular joint, such as a knee, elbow or wrist. An example of a motor driven CPM that is specially adapted for use e.g. in the rehabilitation of a knee or hip joint is disclosed in U.S. patent application Ser. No. 07/760,291, entitled "Continuous Passive Motion Device for a Limb", assigned to the assignee of the present invention and incorporated by reference herein.
In designing a portable continuous passive motion device, the size and/or the power output capacity of the motor is chosen in dependence not only on an actual anticipated load which the device is likely to encounter during operation (such as the weight or the resistance to movement of a leg, arm or hand), but also on the effective "gearing ratio" (or mechanical advantage) which exists between the motor and the limb or joint moving member of the CPM. This effective gearing ratio is a function of the particular motion conversion mechanism which is utilized in the CPM to change the (e.g. rotary) output motion of the motor into the (e.g. oscillatory or reciprocating) output motion of the limb or joint moving member. Moreover, depending on the particular motion conversion mechanism which is utilized, the effective gearing ratio between the motor and the limb or joint moving member may vary e.g. over the range of movement of the limb or joint moving member.
A continuous passive motion device for a wrist is known, for example, from U.S. Pat. No. 5,067,479. This device employs an eccentric transmission for driving a pivoting slide to convert a rotary motion of an electric motor into a pivoting motion of a wrist-supporting tubular shaft driven by the slide. As will become apparent to those skilled in the art, the effective gearing ratio (or mechanical advantage) of such a motion conversion mechanism ranges from a minimum when an axis of the pivoting slide is parallel to an effective working radius of the eccentric transmission (e.g. when the wrist is in an unflexed position) to a near infinite maximum when the axis of the pivoting slide is disposed tangentially to and parallel with the plane of an effective circumference defined by the effective working radius of the rotating eccentric transmission. Due to the relatively large variance in the effective gearing ratio of such a motion conversion mechanism, the selection of a proper size and/or power output capacity for the electric motor is necessarily controlled by countervailing functional and practical design constraints. Specifically, if a relatively large size and/or power output capacity for the electric motor is chosen so as to accommodate the actual anticipated load when the effective gearing ratio is at the minimum, then that motor exhibits a substantial overcapacity in power output when the wrist-supporting tubular shaft is at a position where the minimum effective gearing ratio does not exist. Conversely, if a relatively small size and/or power output capacity for the electric motor is chosen so as to reduce an overcapacity condition in the power output of the motor when the effective gearing ratio is not at a minimum, then the power output capacity of that motor may be inadequate to overcome the actual anticipated load when the wrist-supporting tubular shaft is at a position where the minimum effective gearing ratio exists in the motion conversion device.
While a relatively large motor functions adequately even when the minimum effective gearing ratio exists in the motion conversion device of the CPM, several practical design constraints weigh against the selection of such a motor. Specifically, both the cost and the aesthetic appearance of the CPM are adversely affected by the selection of a relatively large motor. Also, in order to accommodate the larger reaction forces generated by the relatively large motor, the design of the CPM housing necessarily becomes more complex. Additionally, a relatively large motor consumes more electric power than a relatively small motor.
Accordingly, it will become apparent that the selection of the proper size and/or power output capacity of an electric motor for a portable wrist CPM with an eccentric transmission-type motion conversion mechanism involves a balancing of, or alternately, a compromise between, functional and practical design constraints.